Further Development 21.5: Specification and Proliferation

Metamorphosis: The Hormonal Reactivation of Development

Specification of the general cell fates (i.e., that the disc is to be a leg disc and not a wing disc) occurs in the embryo and is mediated primarily by the Hox genes, such as Ultrabithorax and Antennapedia. Increasingly specific cell fates are specified in the larval stages, as the cells proliferate (von Kalm et al. 1995). The type of leg structure (claw, femur, etc.) generated is determined by the interactions between several genes in the imaginal disc. Figure 1 shows the expression of three genes involved in determining the proximal-distal axis of the fly leg. In the third instar leg disc, the center of the disc secretes the highest concentration of two morphogens, Wingless (Wg, a Wnt paracrine factor) and Decapentaplegic (Dpp, a BMP paracrine factor). High concentrations of these paracrine factors induce expression of the Distal-less gene. Moderate concentrations induce the expression of the dachshund gene, and lower concentrations induce the expression of the homothorax gene.

Those cells expressing Distal-less telescope out to become the most distal structures of the leg—the claw and distal tarsal segments. Those expressing homothorax become the most proximal structure, the coxa. Cells expressing dachshund become the femur and proximal tibia. Areas where the transcription factors overlap produce the trochanter and distal tibia (Abu-Shaar and Mann 1998). These regions of gene expression are stabilized by inhibitory interactions between the protein products of these genes and those of the neighboring genes. In this manner, the gradient of Wg and Dpp proteins is converted into discrete domains of gene expression that specify the different regions of the Drosophila leg.

**Figure 1** Expression of transcription factor genes in the *Drosophila* leg disc. At the periphery, the *homothorax* gene (purple) establishes the boundary for the coxa. The expression of the *dachshund* gene (green) locates the femur and proximal tibia. The most distal structures, the claw and distal tarsal segments, arise from the expression domain of *Distal-less* (red) in the center of the imaginal disc. The overlap of *dachshund* and *Distal-less* appears yellow and specifies the distal tibia and trochanter segments. (A–C) Gene expression at successively later stages of pupal development. (D) Localization of expression domains of the genes onto a leg immediately prior to eclosion. The areas where there is overlap between expression domains are shown in yellow, aqua, and orange (A–C).

Literature Cited

Abu-Shaar, M. and R. S. Mann. 1998. Generation of multiple antagonistic domains along the proximodistal axis during Drosophila leg development. Development 125: 3821–3830.
PubMed Link

Von Kalm, L., D. Fristrom and J. Fristrom. 1995. The making of a fly leg: A model for epithelial morphogenesis. BioEssays 17: 693–702.
PubMed Link

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